The present invention relates to thermostats and, in particular, to an electromechanical thermostat incorporating mechanical heat anticipation and control or regulation of the droop characteristics of the thermostat, i.e., the difference between the thermostat set point and the ambient control temperature as a function of the duty cycle of the controlled heating and/or cooling power plant.
Electromechanical thermostats are those of the type having a temperature sensing element that responds mechanically to temperature changes. For example, the sensing element may comprise two dissimilar metals bonded together (bimetals) having unequal rates of expansion. One type of thermostat incorporating this sensing element concept has a bimetal coil with a fixed end and an end which is allowed to move as the bimetal expands in response to changes in temperature. The free end typically actuates a switch, such as a mercury switch, to turn the heating/cooling source on and off. Another example of this type of electromechanical thermostat is one having an elongated bimetal sensing contact fixed at one end with its opposite end moving under the influence of temperature changes into and out of engagement with a switch contact generating the heating/cooling unit on and off signals.
These types of thermostats embody what is referred to as heat anticipation. Heat anticipation is generally accomplished by a resistance heater which provides heat to the temperature sensing element during the time the unit is on during heating (or alternatively, while the unit is off during a cooling cycle). This increased temperature in the sensing element provides anticipation to the rise in ambient temperature causing the thermostat switch to open and turn off the heating unit before the ambient temperature increases sufficiently to break the contact. Without this heat anticipation the ambient temperature swings within the control space would become excessive because of the switch differential and the time lags of the sensing element and the heating system. Heat anticipation thus generates a proper cycling of on and off conditions.
Wireless thermostats may be used in retrofit applications. Typically in these applications the thermostats are not hard wired to circuitry that provides an adequate source of electrical power for a heat anticipation resistance heater. In such applications typically the electrical power available for the thermostat is a self-contained battery. The wireless thermostat incorporates a radio frequency transmitter that provides a communication control signal to a central receiver for heating/cooling unit control. The prior art includes solid state wireless thermostats with battery power operation. In the case of electromechanical thermostats the power necessary to generate heat anticipation in the conventional manner would cause an excessive drain on the self-contained battery.
Heat anticipation thermostats characteristically exhibit a difference between the thermostat set point and the ambient temperature at which the room is controlled. This is referred to as the "droop" characteristic. It occurs because in order to change the rate of heat input a temperature change has to be detected at the sensing element location. In the heating mode the offset or droop in the room control temperature increases as the ambient temperature decreases. Thus, the room becomes less comfortable. It is desirable to reduce or eliminate this droop. In a solid state thermostat as previously described this can be accomplished by detection of the droop of the heating/cooling unit and resetting the thermostat to reduce or eliminate the difference between the controlled temperature and set point. This has not previously been effectively accomplished with an electromechanical thermostatic device.